If copyright is violated, it’s not theft (depriving someone of their work); it’s infringement. When infringement occurs, publishers and authors can seek relief, generally payment for unauthorized use. Changing the law, however (which is what the Authors Guild hopes will happen), should depend on an assessment of adverse impact.

Books uploaded to file-sharing sites are an infringing use. They represent (in my world) an “instance of piracy”. Someone did something with book content they should not have done. Copyright owners can seek remedies.

The real question is not whether piracy occurs (it does). You want to know what effect it has on sales, and ultimately (if you want to change laws related to copyright) you want to establish its impact on creation of works that “promote the progress of science and useful arts”.